Capacitive proximity switch for evaluating minor changes in capacitance and method therefor

ABSTRACT

Invention relates to a capacitive proximity switch with an electrical bridge circuit for detecting of an electrically conducting face (15) approaching or moving away, wherein the electrically conducting face (15) is part of a capacitor (for example C1) of the bridge circuit, wherein at least one capacitor (C1, C2, C3, C4) and possibly at least one resistor (R9, R10) are disposed in the bridge branches of the bridge circuit as further reactances, and wherein the bridge is subjected to an alternating voltage as a bridge feed voltage (ubr). The proximity switch includes a flat multilayer printed circuit board (10) having at least two electrically insulating layers (13, 14), wherein an electrically conducting intermediate layer (11) as a first face of a capacitor is disposed between the two electrically insulating layers (13, 14).

TECHNICAL FIELD

The invention relates to a capacitive proximity switch for theevaluation of capacitance changes with an electrical alternating currentmeasurement bridge of four bridge branches, which reach every two offour switching knots of the bridge, wherein an alternating current ispositioned as a bridge feed voltage near two opposite, not neighboringswitching knots, and respectively the two bridge branches between thetwo feed in switching knots build each of the two halves of the bridge,and there is an evaluated bridge diagonal voltage on the two remainingswitching knots over diagonal voltage path, wherein the bridge diagonalvoltage is removable, for detecting an approaching or moving away face,in particular an electrically bad conductive face or metallic face,which face is part of variable capacitor in one of the bridge branchesof the bridge circuit, wherein at least one capacitor (capacitivebridge) or at least one capacitor and/or one resistor (capacitive ohmicbridge) is disposed in the remaining bridge branches as furtherreactences, wherein rectifiers are arranged to the rectification of theboth bridge branch voltages separately according to respective bridgehalves in the diagonal voltage paths and the diagonal bridge voltage isevaluated only after the rectification of the both bridge branchvoltages as changing direct current corresponding to the capacitancechange of the variable capacitor, according to the preamble of the claimone as well as method according to preamble of the claim 9.

STATE OF THE ART

The evaluation of small up to very small changes of capacitance is analways returning task in sensor technology. Very small changes incapacitance, namely in an order of magnitude of less than 10 fF, have tobe reliably evaluated in particular in connection with capacitiveproximity switches, wherein here in particular the stability againstinterferences as well as the temperature stability of the respectivecircuit assumes a central importance.

It is known to employ an oscillator for determining small changes incapacitance, wherein the oscillating amplitude of oscillator changesdepending on the capacitance of the sensor. The size of the oscillationamplitude thus is a measure for the value of the capacitance of thesensor. Such a circuit is associated with the disadvantage that thecircuit is not stable relative to temperature based on a principle, andtherefore an eventually difficult to dimension temperature compensationis necessary in connection with such a circuit in most cases.Furthermore the quality and efficiency of the oscillator is low,wherefrom a broad band circuit with a bad electromagnetic compatibilitybehavior results.

Furthermore, it is known for the determination of small changes incapacitance to employ an oscillator, wherein the frequency of theoscillator changes depending on the capacity of the sensor. Again thedisadvantage is associated with the employment of such an oscillator,that such a circuit is not stable relative to temperature based on aprinciple, and therefore here again an eventually difficult to dimensiontemperature compensation is necessary; also such circuit has arelatively bad electromagnetic compatibility behavior. so-calledswitched capacitor technique is furthermore known for determining smallchanges in capacitance, wherein a critical timing of the clock cyclesignals is of disadvantage and wherefore an extremely stable clock cyclesignal is required, which imposes an expensive switching technologydepending on the method.

Also integrated circuits are furthermore known for the determination ofsmall changes in capacitance. The integrated circuits exhibit thedisadvantage that in most cases a ground free capacitance is necessary.In addition such circuits require in general a digital evaluation unit(mostly a counter) which means a large expenditure for switchingtechnology. Such concepts are employed predominantly in the microsystems technology for these reasons.

It is furthermore known that the determination of small changes of anelectrical value can be realized advantageously with the bridgecircuits. The value to be measured is here compared with referencevalues, wherein these reference values are generated by similarlyoperating elements. Thus temperature influences can be effectivelysuppressed as long as the two bridge branches of the bridge have at alltimes the same temperature. Changes of the respective value are thenpresented as changes of the bridge diagonal voltage. The employment ofreactances as bridge elements causes that the bridge has to be operatedwith alternating current. Thus also the bridge diagonal voltagerepresents an alternating voltage. The evaluation of the bridge diagonalvoltage presents here frequently a problem according to the state of theart, because

the amplitude of the alternating voltage is very small based on thesmall change in capacitance, namely a few millivolts mV;

the frequency of the alternating voltage, which is employed foroperating the bridge, and therewith also the frequency of the bridgediagonal voltage is located in the MHz region in order to avoid that thebranch currents do not assume too small values given the small capacityvalues;

the bridge diagonal voltage in many cases in addition to the alternatingvoltage component also includes a common mode direct current componentwith a substantial larger as compared to the alternating currentcomponent.

Such previously known solutions are contained for example in the printeddocuments DE-C2-3143114, DE-A1-3911009, DE-A1-19536198, DE-A1-19701899,CH-558534, as well as in the EP-A1-0723166.

A circuit arrangement for eliminating the influence of a phase shiftbetween the voltage potentials of the two measurement points of analternating current measurement bridge with complex resistances isfurnished, wherein the difference of the voltage potentials forms themeasurement signal. A rectifier valve is disposed between themeasurement points and the input enclosures of a circuit for forming thedifference in each voltage path, wherein a storage disposed between theoutput of the rectifier valve and a reference potential common to thealternating current measurement bridge is connected following to each ofthe rectifier valves.

Capacitors employed as capacitive measurement value receivers within analternating current measurement bridge, either in a bridge circuit withfour capacitances or with two capacitances and two resistors, wherein ineach case a capacitance is variable, are known from the literaturelocation Heinz Schneider, Kondensatoren als Messwertaufnehmer(Capacitors as measurement value receivers), Elektronik-Applikation Nr.14, Jul. 9, 1985.

A proximity switch with an alternating current measurement bridge isknown by FR-A-2371676, which is subjected to an alternating current. Thebridge branch voltages are rectified and subject to single electricalmanipulations. Only after the rectification of the bridge branchvoltages these as changing direct current as capacitive size areevaluated.

TECHNICAL PURPOSE

It is an object of the present invention to furnish a capacitiveproximity switch as well as a method, which is highly sensitive to theapproach of the object on one of its sides and by this allow a reliableevaluation of small changes in capacitance, wherein the circuit is toshow a high resistance to interferences as well as a high-temperaturestability based on its principle. Furthermore, proximity switch is to berealizable with comparatively small expenditure and therefore at lowcosts.

DISCLOSURE OF THE INVENTION AND ITS ADVANTAGES

The proximity switch according to the present invention comprises a flatmultilayer circuit board comprising at least two electrically insulatinglayers, wherein an electrically conductingly intermediate player isdisposed between the two electrically insulating layers as a first placeof capacitance in one of the two bridge branches of one of the bridgehalves and wherein a flat electrically conducting covering is placed onone of the two layers furnishing a sensor, wherein the flat coveringforms the second face of the capacitor, wherein the face is disposedmovable relative to the sensor and forms with the sensor a second flatvariable capacitor, and wherein the face and the sensor form onecapacitance and wherein the sensor and the electrically conductingintermediate layer form the second capacitance of one of the two bridgehalves and wherein this construction represents the one half of thebridge, and wherein rectifires are disposed in the diagonal voltagepaths for rectifying the two bridge branch voltages separately accordingto the respective bridge half and wherein the bridge diagonal voltage isevaluated only after the rectification of the two bridge branch voltagesas the direct current changing corresponding to the capacitance changeof the variable capacitor.

The proximity switch and the method are associated with the advantagethat a reliable evaluation of very small capacitance changes ispossible, wherein the circuit exhibits a high stability againstinterferences and a high-temperature stability and wherein the circuitis substantially insensitive relative to coupled in interferences.Similarly the proximity switch can be realized with comparatively smallexpenditures. The advantages comprise in particular:

no alternating voltage has to be evaluated but only a direct voltage.

a slow operational amplifier or, respectively, comparator can beemployed for evaluating the bridge diagonal voltage.

the rectification of the bridge branch voltages can be performed withdiodes, whereby the very simple circuit with only a few device elementsresults; advantageously a so-called slow diode can be employed in orderto suppress interferences. Alternatively rectification is performedsynchronous with controlled switches, in case a particularly highsuppression of interferences is required.

It is important that the rectifiers of the two branches exhibit the sametemperature behavior in order for the rectified bridge diagonal voltageto be independent of temperature. It is advantageous for goodsuppression of interferences that the switching construction isfurnished symmetrical, because an interference operating in the same wayonto the two bridge branches does not cause the difference voltage atthe rectifier outputs.

The rectification of the bridge branch voltages can be performed eitherby diodes, preferably four diodes, or by controlled switches, preferablyfour controlled switches, wherein in case of the employment of switchesthese switches are controlled pairwise in opposite phase and areswitched synchronously with the bridge voltage from one switching stateto the other.

In addition coupled in interferences can be suppressed effectively, byconnecting and switching in each case a low pass filter, a LP-filter, infront of the rectifier of the bridge branch voltages. It is to beobserved that the capacitance of the LP-filter can be formed byparasitic capacitances of the rectifier elements, for example barrierlayer capacitances or, respectively, diffusion capacitances of thePN-transitions of rectifier diodes for or of the input capacitance ofelectronic switches, such that also the capacitors can be dispensedwith. The electromagnetic compatibility behavior can be improved by theemployment of relatively slow rectifier diodes. In this case possibly anadditional low pass LP-filtering can be dispensed with.

Also four switches with pairwise opposite phase driving for rectifyingthe bridge branch voltages can be employed instead of the four diodes.It is thereby possible to rectify also alternating voltages, wherein theamplitude of the alternating voltages is smaller as compared with thethreshold voltage of a rectifier diode. Furthermore, interferingvoltages are better suppressed because only such alternating voltagesare completely rectified, where the alternating voltages aresynchronized with the switching signal of the switch. In order todecrease the probability that the interfering signal exhibits in thesame frequency as the switching signal of the switch, it is recomendedto change continuously the frequency of the bridge voltage and therebyalso the frequency of the change signal.

Also two transfer switches can be employed instead of the fourindividual driven switches with pairwise opposite phase driving.

The face approaching or moving away, which is part of a capacitor of thecapacitive proximity switch, can also be grounded.

According to the method, the two bridge branch voltages are rectifiedseparately according to the respective bridge half either by four diodesor by four controlled switches as rectifires in the diagonal voltagepaths, wherein the bridge diagonal voltage is driven synchronous withthe bridge feed voltage pairwise in opposite phase upon employment ofswitches which are opened and closed synchronously with the bridge feedvoltage pairwise by means of control voltage as well as opposite phasecontrolled and are switched synchronously with the bridge feed voltagefrom one switching state into the other switching state. The frequencyof the bridge feed voltage and also of the transfer switch signal can bechanged continuously.

The two bridge branch voltages are rectified separately according to therespective bridge half either by four diodes or by four controlledswitches as rectifires in the diagonal voltage paths for evaluatingsmall changes of capacitance under employing of a capacitive proximityswitch, wherein the bridge diagonal voltage is evaluated only after therectification of the two bridge branch voltages as a direct voltagechanging corresponding to the change of the capacitance, and wherein thebridge diagonal voltage is driven synchronous with the bridge feedvoltage pairwise in opposite phase upon employment of switches andwherein the bridge diagonal voltage is switched from one switching stateinto the other switching state synchronous with the bridge voltage. Thefrequency of the bridge feed voltage and also of the transfer switchsignal can be changed continuously.

A balancing of the circuit or, respectively, of the proximity switch isadvantageously performed by changing the capacitance of one of thecapacitors in one of the bridge branches of the bridge. This can beperformed with the aid of a so-called variable tuning capacitor or of alaser trimmed capacitor. It is advantageous to perform the adjustmentand balancing such that the difference voltage is equal to zero in theswitching point of the proximity switch, since it is then sufficient toevaluate only the sign of the difference voltage. If only the sign ofthe difference voltage Ud is evaluated, then there results an outputsignal with the two different states, wherein the switching point atwhich the sign of the difference voltage Ud changes from one state intothe other depends only on the capacitance value of the variable tuningcapacitor, however, not on the amplitude or the frequency of the bridgevoltage ubr or on the size of the forward flow voltage Uf of therectifier diodes.

If it is intended to dispense with the balancing with the aid of avariable tuning capacitor, then the zero point of the difference voltagecan also be set by having two of the rectifier elements with their oneconnection not connected to the reference potential, for example ground,but in each case to a reference voltage source, wherein the value of thereference voltage source is set such that the desired difference voltageUd, that is in most cases zero, is set at the output. If the tworeference voltages are derived such from the bridge supply voltage ubr,that a linear connection exists between the respective reference voltageand the bridge supply voltage ubr, then at change of the bridge supplyvoltage ubr does not affect the bridge diagonal voltage Ud—and thus thebalancing—.

Short description of the drawing, where there is shown:

FIG. 1 a principle of or circuit diagram of a capacitive bridge circuitwith evaluation according to the present invention.

FIG. 2 to the principle of a circuit diagram of FIG. 1 supplemented byin each case a low pass filter in front of the rectifiers.

FIG. 3 a principle of a circuit diagram, wherein the rectifier diodesare replaced by switches.

FIGS. 4a, b, c the replacement of two associated rectifier diodes by twoassociated rectifier and switches or, respectively, the replacement oftwo associated switches by in each case a transfer switch.

FIG. 5 a principle of a circuit diagram for setting the zero point ofthe bridge diagonal voltage with two reference voltage sources.

FIG. 6 the employment of diodes with the threshold voltage Uf asrectifying elements in FIG. 5.

FIG. 7 a possibility for generating the reference voltage by rectifyingbridge feed voltage and two voltage dividers.

FIG. 8 the circuit diagram of FIG. 7 stranded by N5, N6, R8, C10, and Uvfor showing that the rectified voltage Ugl is independent of the forwardflow voltage Uf of the rectifier diodes.

FIG. 9 a further principle of a circuit diagram of a bridge circuit within each case a capacitance disposed in the one bridge half and an ohmicresistor in each case in the other bridge half, and

FIG. 10 a principle of a construction of a capacitive proximity switchwith the formation of the capacitances of one bridge branch by differentlayers of amount the layer circuit board in combination with theirenvironment.

The same elements are furnished with the same reference characters inthe figures. For clarification the following terms are employed: ‘bridgehalf’ means—graphically in the figures—the left or, respectively righthand side or, respectively, half of the bridge, with ‘bridge branch’being the switching part between the two switching knots such that abridge includes two ‘halves’ and four ‘branches’ or, respectively,‘bridge branches’.

PATHS FOR PERFORMING THE INVENTION

FIG. 1 shows a principle circuit diagram of a capacitive electricalbridge circuit according to the present invention, such as capacitiveelectrical bridge circuit can be employed for constructing of acapacitive proximity switch comprising the bridge with in each case acapacitance C1, C2, C3, and C4 in each bridge branch; the capacitance C1is variable. The bridge is fed by the bridge feed voltage ubr, whereinthe bridge feed voltage is an alternating voltage; the referencepotential is ground GND.

The two bridge branch voltages uc1 and uc3 from the two bridge halvesare rectified separately according to the respective bridge branch withthe diodes N2, N1 or, respectively, N4, N3 disposed in the diagonalvoltage paths DSp1 and DSp2 and are smoothed in each case relative toground with capacitors C5, C6 as well as resistances R1 and R2,whereupon the bridge diagonal voltage Ud is evaluated as a directvoltage changing corresponding to the change of the capacitance C1. Plusthe bridge diagonal voltage of the diagonal voltage paths DSp1 and DSp2is obtained only after rectification of the two bridge branch voltagesuc1 and uc3 as a difference voltage Ud, that means according to FIG. 1as a rectified voltage U out of udiff+−udiff−.

FIG. 2 shows the principle circuit diagram of FIG. 1 supplemented ineach case by a low pass filter in each bridge branch in front of therectifires N2, N1, N4, N3 for suppressing of coupled in interferences,formed by the resistances R3 and R4 disposed in the diagonal paths Dsp1and DSp2 for coupling out of the two bridge branch voltages uc1 and uc3as well as the capacitors C7, C8. The capacitors C7 and C8 of the lowpass filter R3-C8 as well as R4-C7 can also be formed by the parasiticcapacitances of the elements of the rectifier N1, N2, N3, N4.

FIG. 3 shows a further principle circuit diagram, wherein the rectifierdiodes are replaced by electronic switches S2, S1 or, respectively, S4,S3. The two switches S2, S4 disposed in the respective de-coupling linesof the bridge branches are addressed and closed as well as opened by thecontrol voltage Ust2, the two switches S1, S3 disposed against groundare addressed and closed as well as opened by the control voltage Ust1.These switches are pairwise opposite phase controlled. The capacitancesof the respective low pass filters can be formed by parasiticcapacitances of the elements of these switches S1, S2, S3, S4 even uponemployment of switches and low pass filters.

FIG. 5 shows a principle circuit diagram for setting for example thezero point of the bridge diagonal voltage Ud with two reference voltagesources Uref1 and Uref2. Two of the rectifier elements—diodes orswitch—are connected with their one connector not to ground but in eachcase to a reference voltage source Uref1, Uref2 for setting of the zeropoint of the bridge diagonal voltage Ud, wherein the reference potentialis ground GND of the reference voltage sources Uref1, Uref2. The valuesof the reference voltage source Uref1, Uref2 are adjusted such that thedesired bridge diagonal voltage Ud is set at the output.

FIG. 6 shows the employment of diodes as rectifier elements in FIG. 5,wherein the diodes exhibit the threshold voltage Uf. Then they result ofthe following relationships:

Ud=U 1−U 2

The capacitive voltage dividers of the two bridge branches weaken thesignal ubr by the factor k or, respectively k′. If the cathodes of tworectifier diodes are connected to the reference potential, that is toground, that is in case Uref1=Uref2=0, then one obtains after therectification the voltages

 toU 1=kubr−2Uf or, respectively, U 2=k′ubr−2Uf

Here ubr is the peak-peak-value of the bridge feed voltage. If incontrast the diodes are connected on the cathode side in each case to areference voltage different from zero, then one obtains

U 1=kubr−2Uf+Uref 1 or, respectively U 2=k′ubr−2Uf+Uref 2

One then obtains for the difference voltage Ud:

Ud=U 1−U 2=kubr−2Uf+Uref 1−k′ubr+2Uf−Uref 2=ubr(k−k′)+Uref 1−Uref 2

It can recognize from this that for k′=k and for Uref2=Uref1 then thevoltageUd becomes zero and in fact independent of the size of the bridgefeed voltage ubr. If however k and k′ have different values, then thereresults a difference voltage Ud different from zero. The differencevoltage Ud can be made in fact 20 by a suitable choice of Uref1 andUref2. A disadvantage is however associated with the situation that thedescribed zero balancing depends on the bridge voltage ubr. Therefore itwould be necessary in this case to maintain the amplitude of the voltageubr at a steady value. This is possible in particular in view oftemperature changes in general only under substantial expenditures.

FIGS. 7 and 8 show further developments of switching circuits forgenerating of the reference voltage by rectification of the bridge feedvoltage and two voltage dividers for generating of the about describezero balance independent of the bridge voltage ubr. For this purpose thetwo reference voltages Uref1 and Uref2 are derived from the bridge feedvoltage ubr at the switching point A, compare FIGS. 6 and 7, and in factin the shape that a linear connection exists between the amplitude ofthe bridge voltage and the reference voltage in each case. It holds thenfor the two reference voltages:

Uref 1=pubr or, respectively, Uref 2=p′ubr

There results then for the difference voltage Ud the followingrelationship:

Ud=ubr(K′−k)+P′ubr−pubr=ubr(k′−k+p′−p)

If p′ andp are then selected such that holds:

k′−k=p−p′

Then the difference voltage Ud becomes zero independent of the amplitudeof the bridge voltage ubr. A simple possibility for generating thereference voltage according to the above recited relationship comprisesto rectify the bridge voltage ubr and to set the bridge voltage to thedesired value with in each case of a voltage divider, which is just whatis shown in FIG. 7.

FIG. 8 shows a switching circuits, wherein the rectified voltage Ugl isalso independent of the forward voltage Uf of the rectifier diodes. Therectified bridge voltage is also temperature dependent based on thetemperature dependence of the forward voltage of the rectifier diodes.

The following relationship holds for the rectified voltage Ugl:Ugl=ubr−2Uf.

If the cathode of the rectifier diodes is not connected to the referencepotential, that is to ground, but instead to a potential of the heightlevel 2Uf, then the equation simplifies the following expression:Ugl=Ubr.

The rectified voltage is thus independent of the forward voltage Uf.

In principle the circuit according to the present invention can also beconstructed with a bridge, wherein the bridge comprises two capacitorsand two resistors in each bridge branch, which is shown in principle inFIG. 9. A capacitive bridge circuit with in each case a capacitance C1and C2 as well as in each case an ohmic resistor R9, R10 in each bridgebranch is shown in FIG. 9. The corresponding bridge branch voltage hasto be decoupled through a capacitor C11 in the bridge half constructedwith the resistors R9, R10, otherwise the bridge corresponds to theabout describe FIG. 1.

The describe circuit arrangement is in particular suited for theconstruction of the capacitive proximity switch for detecting of a face15 approaching or moving away, for example of an electrically badconductive face or a metal face. FIG. 10 shows a principal constructionof such a capacitive proximity switch, which represents ½ of the bridge,with the formation of capacitances over bridge branch by various layersof a flat multilayer circuit board 10 in combination with itssurroundings.

The multilayer printed circuit board 10 comprises at least twoelectrically insulating layers 13, 14, wherein an electricallyconducting intermediate layer 11, for example a metallic intermediatelayer, is disposed as a first face of a capacitor between theelectrically insulating layers 13, 14. A flat, also electricallyconducting covering is applied as a sense of 12 on one of the two layers13 or, respectively, 14, namely here of the upper layers 13, wherein thesensor 12 forms the second face of the capacitor, wherein the face 15,that is in particular an electrically bad conductive face or metal face,is disposed movable relative to the sensor 12 and wherein the face 15together with the sensor 12 forms the second flat, variable capacitorwith for example air as a dielectric. The face 15 can also be groundedby way of a conduit 17. The face 15 together with the sensor 12 for mycapacity of the one bridge half, the sensor 12 and the metallic layer 11form the second capacitance of the same bridge half; this constructionrepresents the one half of the bridge. Further electrical deviceelements 16 are placed on the side of the multilayer printed circuitboard 10 disposed opposite to the sensor 12 and to thus on the lowerelectrically insulating layer 14.

At differently large change of the bridge diagonal voltage Du results bythe construction illustrated in FIG. 10, depending on from which side aface, for example an electrically bad conductive face or the metallicface 15, approaches the multilayer printed circuit board 10. Thus oneside of the printed circuit board can operate as a sensor, namely inthis case the upper side with the electrically conducting sensor 12,whereas the second side of the printed circuit board, here the layer 14,is insensitive relative to an approach over face, even then when theconducting face is grounded.

The capacitor face formed by the electrically conducting intermediatelayer 11 can also be exchanged by a discrete capacitor, which discretecapacitor is connected with a connector to the sensor 12. However, inthis case there results and nearly equal sized change of the bridgediagonal voltage Ud, independent of it from which side an electricalconducting face approaches or moves away from the sensor 12. Anintermediate layer or a shielding, such as for example the electricallyconducting intermediate layer 11, therefore should always then beprovided in case the proximity switch is thinner in its thicknessconstruction as the region of influence of the proximity switch.

Thus in a simple way and under favorable costs it is possible to producecapacitive proximity switches in a flat construction shape, wherein thecapacitive proximity switches can also be mounted onto a grounded metalpart.

COMMERCIAL APPLICABILITY

The subject matter of the present invention is suitable in particularfor employment as a highly sensitive capacitive proximity switch. Theusefulness of the proximity switch according to the present inventioncomprises in particular that the proximity switch is on the one sidehighly sensitive relative to the approach of an object, however, is forpractical purposes insensitive against such an approach on theoppositely disposed side.

What is claimed is:
 1. A capacitive proximity switch for the evaluationof capacitance changes with an electrical alternating currentmeasurement bridge of four bridge branches, which reach every two offour switching knots of the bridge, wherein an alternating current ispositioned as a bridge feed voltage (ubr) near two opposite, notneighboring switching knots, and respectively the two bridge branchesbetween the two feed in switching knots build each of the two halves ofthe bridge, and there is an evaluated bridge diagonal voltage on the tworemaining switching knots over diagonal voltage path (DSp1, DSp2),wherein the bridge diagonal voltage (Ud) is removable, for detecting anapproaching or moving away face, in particular an electrically badconductive face or metallic face, which face is part of variablecapacitor (C1) in one of the bridge branches of the bridge circuit,wherein at least one capacitor(C1,C2,C3,C4)(capacitive bridge) or atleast one capacitor(C1,C2,C3,C4) and/or one resistor (R9,R10)(capacitive ohmic bridge) is disposed in the remaining bridge branchesas further reactances, wherein rectifiers (N1,N2,N3,N4,S1,S2,S3,S4) arearranged to the rectification of two bridge branch voltages (uc1,uc3)separately according to respective bridge halves in the diagonal voltagepaths (DSp1, DSp2) and the diagonal bridge voltage (Ud) is evaluatedonly after the rectification of the two bridge branch voltages (uc1,uc3)as changing direct current corresponding to the capacitance change ofthe variable capacitor, characterized in that, a) the proximity switchcomprises a flat multilayer printed circuit board (10) of at least twoelectrically insulating layers (13,14), wherein an electricallyconducting intermediate layer (11) as a first face of a capacitor in oneof the two bridge branches of one of the bridge halves is disposedbetween the at least two electrically insulating layers (13,14) b) atone of the layers (13,14) at the opposite side of the intermediate layer(11) a flat electrically conducting covering is placed onto one of thetwo layers (13,14) as a sensor (12), wherein the electrically conductingcovering forms the second face of the capacitor c) the face (15) ismobile relative to the sensor (12) and the face (15) forms together withthe sensor (12) a flat variable capacitor of the bridge circuit. 2.Proximity switch according to claim 1 characterized in that therecifiers in the diagonal voltage paths (DSp1, DSp2) either diodes (N1,N2, N3, N4) or controlled switches (S1, S2, S3, S4), are preferably fourin each case.
 3. Proximity switch according to claim 2 characterized inthat the switches (S1,S2,S3,S4) are opened and closed synchronously withthe bridge feed voltage (ubr) pairwise (S1,S2,S3,S4) by means of controlvoltage (Ust1,Ust2) as well as opposite phase controlled and areswitched synchronously with the bridge feed voltage (ubr) from oneswitching state into the other switching state.
 4. Proximity switchaccording to claim 3, characterized in that in each case a low passfilters (R3, C8, and R4, C7) are connected and switched in front of therectifires (N1, N2, N3, N4, or, respectively, S1,S2,S3,S4) of the bridgebranch voltages (uc1, uc3).
 5. Proximity switch according to claim 4,characterized in that the capacitances of the low pass filters (C8 or,resecptively C7) are formed by the parasitic capacitances of therectifier elements (N1, N2, N3, N4 or, respectively,S1, S2, S3, S4). 6.Proximity switch according to claim 2, characterized in that twotransfer switches (W1, W2) are employed as switches for rectifying ofthe bridge branch voltages (uc1, uc3).
 7. Proximity switch according toclaim 1, characterized in that the capacitor face formed by theelectrically conducting intermediate layer (11) is replaced by adiscrete capacitor, wherein the discrete capacitor is connected with aconnection to the sensor (12).
 8. Proximity switch according to claim 2,characterized in that the diodes (N1, N2, N3, N4) are slow diodes forproving the stability against interferences.
 9. A method for theevaluation of capacitance changes with an electrical alternating currentmeasurement bridge of four bridge branches, which reach every two offour switching knots of the bridge, wherein an alternating current ispositioned as a bridge feed voltage (ubr) near two opposite, notneighboring switching knots, and respectively the two bridge branchesbetween the two feed in switching knots build each of the two halves ofthe bridge, and there is an evaluated bridge diagonal voltage on the tworemaining switching knots over diagonal voltage path (DSp1, DSp2),wherein the bridge diagonal voltage (Ud) is removed, for detecting anapproaching or moving away face, in particular an electrically badconductive face or metallic face, which face is part of variablecapacitor (C1) in one of the bridge branches of the bridge circuit,wherein at least one capacitor(C1,C2,C3,C4) (capacitive bridge) or atleast one capacitor(C1,C2,C3,C4) and/or one resistor (R9,R10)(capacitive ohmic bridge) is disposed in the remaining bridge branchesas further reactances, wherein the bridge diagonal voltage (Ud) isevaluated only after rectification of both bridge branch voltages(uc1,uc3) as changing direct current corresponding to the capacitancechange of the variable capacitor, characterized in that, a) the twobridge branch voltages(uc1, uc3) are rectified separately according tothe respective bridge half either by four diodes(N1,N2,N3,N4) or by fourcontrolled switches(S1,S2,S3,S4) as rectifiers in the diagonal voltagepaths(Dsp1,Dsp2) b) the bridge diagonal voltage is driven synchronouslywith the bridge feed voltage (ubr) pairwise (S1,S2,S3,S4) uponemployment of switches which are opened and closed synchronously withthe bridge feed voltage pairwise by means of control voltage (Ust1,Ust2)as well as opposite phase controlled and are switched synchronously withthe bridge feed voltage from one switching state into the otherswitching state c) the frequency of the bridge feed voltage (ubr) andthereby also of the transfer switching signal of the switches(S1,S2,S3,S4) is changed continuously.
 10. Method according to claim 9characterized in that a capacitance of a variable capacitor (C2, C3, C4)is changed such in one of the bridge branches for balancing of thebridge and for setting of the zero point of the bridge diagonal voltage(Ud), that the bridge diagonal voltage (Ud) is equal to zero in theswitching point of the proximity switch and only the sign of the bridgediagonal voltage (Ud) is evaluated.
 11. Method according to claim 9characterized in that two of the rectifier elements(N1, N2, N3, N4, S1,S2, S3, S4) are connected to with their one connector in each case to areference voltage source (Uref1, Uref2) for setting of the zero point ofthe bridge diagonal voltage (Ud), wherein the respective value of thereference voltage source (Uref1, Uref2) is adjusted such that thedesired bridge diagonal voltage (Ud) of the zero volts (0V) is set atthe output.
 12. Method according to claim 11 characterized in that thetwo reference voltages (Uref1, Uref2) are derived and rectified from thebridge feed voltage (ubr) for the balancing of the bridge such that alinear relationship exists between the amplitude of the bridge feedvoltage (ubr) and the respective reference voltage (Uref1, Uref2). 13.Method according to claim 12, characterized in that the referencevoltages (Uref1, Uref2) is set to the desired values with the aid of ineach case a voltage divider.
 14. A capacitive proximity switch for theevaluation of capacitance changes comprising: an electrically conductingintermediate layer (11) forming a first face of a fixed capacitor; anelectrically insulating upper layer (13) disposed at an upper side ofthe electrically conducting intermediate layer (11); a flat electricallyconducting covering placed onto the electrically insulating upper layer(13) for forming a sensor (12), wherein the electrically conductingcovering forms a second face of the fixed capacitor and wherein theelectrically conducting covering forms a first face of a flat variablecapacitor (C1); an electrically insulating lower layer (14) disposed ata lower side of the intermediate layer (11), wherein the electricallyconducting intermediate layer (11), the electrically insulating upperlayer (13), the flat electrically conducting covering, and theelectrically insulating lower layer (14) form a flat multilayer printedcircuit board (10); a mobile face (15), wherein the mobile face (15) ismovable relative to the sensor (12) and wherein the mobile face (15)together with the sensor (12) forms the flat variable capacitor (C1) ofthe bridge circuit; an electrical alternating current measurement bridgecircuit of four bridge branches, wherein a first bridge branch includesthe variable capacitor (C1) and wherein a second bridge branch includesthe fixed capacitor, and wherein the bridge branches each reach two offour switching knots of the electrical alternating current measurementbridge circuit; a current source furnishing an alternating current andpositioned as a bridge feed voltage (ubr) near two opposite, notneighboring feed-in switching knots, and wherein two bridge branchesbetween the two feed-in switching knots form each of two halves of theelectrical alternating current measurement bridge, and wherein anevaluated bridge diagonal voltage is present on two remaining switchingknots over a diagonal voltage path (DSp1, DSp2), wherein the bridgediagonal voltage (Ud) is picked up for detecting an approaching ormoving away mobile face (15), and in particular of an electrically badconductive face or metallic face, which mobile face (15) is part of thevariable capacitor (C1) in the first bridge branch of the electricalalternating current measurement bridge circuit; at least onecapacitor(C1,C2,C3,C4)(capacitive bridge) or at least onecapacitor(C1,C2,C3,C4) and one resistor (R9,R10) (capacitive ohmicbridge) disposed in the remaining bridge branches not containing thevariable capacitor (C1) as further reactance; rectifiers(N1,N2,N3,N4,S1,S2,S3,S4) arranged and connected to the electricalalternating current measurement bridge for furnishing a rectification ofthe two bridge branch voltages (uc1,uc3) separately according torespective bridge halves in the diagonal voltage paths(DSp1, DSp2) andwherein a diagonal bridge voltage (Ud) is evaluated only after therectification of the two bridge branch voltages (uc1,uc3) as changingdirect current corresponding to the capacitance change of the variablecapacitor (C1).
 15. (new) A method for evaluating changes of capacitancechanges comprising employtg an electrical alternating cuntnt measurementbridge of four bddge branches, which bridge branches reach every two offour switching knots of the bridge; using an alternating current andpositioning the alternating current as a bridge feed voltage (ubr) neartwo opposite, not neighboring switching knots, and respectively the twobridge branches between the two feed in switching knots build each ofthe two halves of the bridge; placing mi evaluation bridge diagonalvoltage on two remaining switching knots over a diagonal voltage pat(DSp1, DSp2); removing the bridge diagonal voltage (Ud) for detecting anapproaching or moving away face, which face is part of variablecapacitor (C1) in one of the bridge branches of the bridge circuit;disposing at least one capacitor(CI,C2,C3,C4)(capacitive bridge) or atleast one capacitor(C1,C2,C3,C4) and/or one resistor (R9,R10)(capacitive ohmic bridge) in remaining bridge branches as &rtherreactances; rectifying separately two bridge branch voltages(ucl,uc3)according to the respective bridge half either by four diodes(NI,N2,N3,N4) or by four controlled switches(S1,S2,S3,S4) disposed asrectifiers in the diagonal voltage paths(Dsp1,Dsp2); evaluatiiu a bridgediagonal voltage (Ud) only afier rectification of the two bridge branchvoltages (ucl,uc3) as changing direct current corresponding to acapacitance change of the variable capacitor; synchromusly driving thebridge diagonal voltage with the bridge feed voltage (ubr) pairwise(S1,S2,S3.S4) upon employment of switches; opening and closing theswitches synchronously with the bridge feed voltage psirwise by means ofcontrol voltage (Ust1,Ust2); controlling with an opposite phase andswitching synchronously the switches with the bridge feed voltage (ubi)from one switching state into the other switching state; andcontinuously changing the frequency of the bridge feed voltage (ubr) andthereby also of the transfer switching signal of the switches(S1,S2,S3,S4).
 16. The method according to claim 15 further comprisingchanging a capacitance of a variable capacitor (C2, C3, C4) such in oneof the bridge branches for balancing of the bridge and for setdng of thezero point of the bridge diagonal voltage (Ud), that the bridge diagonalvoltage (Ud) is equal to zero in the switching point of the proximityswitch and only the sign of the bridge diagonal voltage (Ud) isevaluated.
 17. The method according to claim 15 further comprisingconnecting two of the rectifier elements(N1, N2, N3, N4, S1, S2, S3, S4)with their one connector in each case to a reference voltage source(Uref1, Uref2) for setting of the zero point of the bridge diagonalvoltage (Ud); and adjusting the respective value of the referencevoltage source (Ureff, Uref2) such that the desired bridge diagonalvoltage (lid) of tern volts (OV) is set at an output.
 18. The methodaccording to claim 17 further comprising deriving and rectifying the tworeference voltages (Urefl, Uref2) from the bridge feed voltage (ubr) forthe balancing of the bridge such that a linear relationship existsbetween the amplitude of the bridge feed voltage (ubr) and therespective reference voltage (Uref1, Uref2).
 19. The method according toclaim 18 further comprising setting the reference voltages (Uref1,Uref2) to desired values with the aid of in each case a voltage divider.